In the above example, we demonstrated that the value y (above the
table in the mfx output) is the value you obtain
when you predict at the average values of the covariates and the average
value of the offset. In the next example, we will go further and calculate
the marginal effects of two dichotomous variables by hand:

The above example gives you the idea of what to do if you want to evaluate
marginal effects at a value of the offset that is not the mean. You can
replace the offset by the value you want before running
mfx, as follows:

Let’s finish with a more complicated prediction option. By
differentiating the formula for the probability of success, we can verify
that mfx is correctly calculating the marginal
effect. The formula for the probability of success is in [R]
hetprob: